U.S. patent application number 17/274881 was filed with the patent office on 2022-02-24 for cryogenically cooled vacuum chamber radiation shields for ultra-low temperature experiments and extreme high vacuum (xhv) conditions.
The applicant listed for this patent is The Regents of the University of Colorado, a body corporate. Invention is credited to Daniel Dessau, Justin Griffith.
Application Number | 20220057046 17/274881 |
Document ID | / |
Family ID | |
Filed Date | 2022-02-24 |
United States Patent
Application |
20220057046 |
Kind Code |
A1 |
Dessau; Daniel ; et
al. |
February 24, 2022 |
CRYOGENICALLY COOLED VACUUM CHAMBER RADIATION SHIELDS FOR ULTRA-LOW
TEMPERATURE EXPERIMENTS AND EXTREME HIGH VACUUM (XHV)
CONDITIONS
Abstract
Methods, systems, and devices for ultra or extreme-high vacuum
are described. Such systems may comprise a vacuum chamber, a target
within the vacuum chamber, two or more overlapping radiation
shields arranged within an inner vacuum space of a vacuum chamber,
and surrounding at least a portion of the target, a first and a
second cooling element unit thermally coupled to a first and second
radiation shield of the two or more overlapping radiation shields,
wherein the first unit is configured to reduce the first radiation
shield's temperature to at least <100K, and the second unit is
configured to reduce the second radiation shield's temperature to
at least <25K, and a third cooling element unit coupled to the
target and isolated from the first and second radiation shield,
wherein the third cooling element unit is configured to reduce the
target's temperature to at least <4K.
Inventors: |
Dessau; Daniel; (Boulder,
CO) ; Griffith; Justin; (Morrison, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Regents of the University of Colorado, a body
corporate |
Denver |
CO |
US |
|
|
Appl. No.: |
17/274881 |
Filed: |
September 12, 2019 |
PCT Filed: |
September 12, 2019 |
PCT NO: |
PCT/US19/50871 |
371 Date: |
March 10, 2021 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62730233 |
Sep 12, 2018 |
|
|
|
62838999 |
Apr 26, 2019 |
|
|
|
International
Class: |
F17C 3/08 20060101
F17C003/08; F25B 9/10 20060101 F25B009/10; F25D 19/00 20060101
F25D019/00 |
Claims
1. An ultra-high vacuum (UHV) or extreme-high vacuum (XHV) system
comprising: a vacuum chamber; a target within the vacuum chamber;
two or more overlapping radiation shields arranged within an inner
vacuum space of a vacuum chamber, wherein the two or more
overlapping radiation shields surround at least a portion of the
target; a first cooling element unit thermally coupled to a first
radiation shield of the two or more overlapping radiation shields,
wherein the first cooling element unit is configured to reduce the
first radiation shield's temperature to at least <100K; a second
cooling element unit thermally coupled to a second radiation shield
of the two or more overlapping radiation shields, wherein the
second cooling element unit is configured to reduce the second
radiation shield's temperature to at least <25K; and a third
cooling element unit thermally coupled to the target, the third
cooling element unit thermally isolated from both the first
radiation shield and the second radiation shield, wherein the third
cooling element unit is configured to reduce the target's
temperature to at least <4K.
2. The system of claim 1, wherein the first cooling element unit
and the second cooling element unit each comprise one or more cold
heads, or two stages of a single cold head.
3. The system of claim 1, wherein the two or more radiation
shields, individually or in combination, cover at least 90% 4.pi.
steradians around the target.
4. The system of claim 1, wherein the second radiation shield
comprises: a sorbent material affixed to an inner surface of the
second radiation shield, wherein the sorbent material is configured
to increase an effective surface area of the second radiation
shield.
5. The system of claim 4, wherein the second cooling element unit
is configured to reduce the second radiation shield's temperature
to at least <15K.
6. The system of claim 1, wherein the second cooling element unit
is switched on a duration of time after the first cooling element
unit is switched on, wherein the duration of time is based at least
in part on a partial pressure of one or more gases in the vacuum
chamber.
7. The system of claim 1, wherein the third cooling element unit is
a dual stage closed-cycle cold head, and wherein a first cold head
of the third cooling element unit is thermally coupled to a third
radiation shield surrounding the target and a second cold head of
the third cooling element unit is thermally coupled to the
target.
8. The system of claim 7, wherein the first cold head and the
second cold head are switched on at different times.
9. The system of claim 1, further comprising: one or more high
permeability shields arranged within the inner vacuum space of the
vacuum chamber, wherein the one or more high permeability shields
surround the first and second radiation shield.
10. A method for ultra-high vacuum (UHV) or extreme high vacuum
(XHV), comprising: providing two or more overlapping radiation
shields within an inner vacuum space of a vacuum chamber, the two
or more overlapping radiation shields covering at least 90% 4.pi.
steradians around a target; thermally coupling a first cooling
element unit to a first of the two or more overlapping radiation
shields; thermally coupling a second cooling element unit to a
second of the two or more overlapping radiation shields; thermally
coupling a third cooling element unit to the target, the third
cooling element unit thermally isolated from the first and second
radiation shields; cooling the first radiation shield to <100K;
cooling the second radiation shield to <25K; cooling the target
to <4K; and interacting an elongated tool with the target
through one or more apertures in the first and second radiation
shields, while maintaining the at least 90% 4.pi. steradians
coverage around the target.
11. The method of claim 10, wherein the first cooling element unit
and the second cooling element unit each comprise one or more cold
heads, or two stages of a single cold head.
12. The method of claim 10, wherein the second radiation shield
comprises a sorbent material affixed to an inner surface of the
second radiation shield.
13. The method of claim 10, wherein the second radiation shield is
cooled to <15K.
14. The method of claim 10, wherein the first and second cooling
element units are switched on at different times.
15. An apparatus for ultra-high vacuum (UHV) or extreme high vacuum
(XHV), comprising: two or more overlapping radiation shields within
an inner vacuum space of a vacuum chamber, wherein the two or more
overlapping radiation shields surround at least a portion of a
target thereby blocking a majority of blackbody radiation from
reaching the target; means for reducing a temperature of the first
radiation shield to <100K; means for reducing a temperature of
the second radiation shield to <25K; means for reducing a
temperature of the target to <4K, and wherein the means for
reducing the temperature of the target is thermally isolated from
both of the means for reducing the temperature of the first
radiation shield and the means for reducing the temperature of the
second radiation shield; and means for interacting with the target
via one or more apertures in the first and second radiation
shields.
16. The apparatus of claim 15, wherein the second radiation shield
comprises a sorbent material on an inner surface of the second
radiation shield.
17. The apparatus of claim 15, wherein the means for reducing the
temperature of the second radiation shield is configured to reduce
the temperature of the second radiation shield to <15K.
18. The apparatus of claim 15, wherein the means for reducing the
temperature of the first radiation shield is turned on before
turning on the means for reducing the temperature of the second
radiation shield.
19. The apparatus of claim 15, wherein the apertures are shaped to
maintain at least 90% 4.pi. steradians of radiation coverage around
the target.
Description
CLAIM OF PRIORITY UNDER 35 U.S.C. .sctn. 119
[0001] The present Application for Patent claims priority to
Provisional Application No. 62/730,233 entitled "CRYOGENICALLY
COOLED VACUUM CHAMBER RADIATION SHIELDS FOR ULTRA-LOW TEMPERATURE
EXPERIMENTS AND EXTREME HIGH VACUUM (XHV) CONDITIONS" filed Sep.
12, 2018, and Provisional Application No. 62/838,999 entitled
"CRYOGENICALLY COOLED VACUUM CHAMBER RADIATION SHIELDS FOR
ULTRA-LOW TEMPERATURE EXPERIMENTS AND EXTREME HIGH VACUUM (XHV)
CONDITIONS" filed Apr. 26, 2019, both of which are assigned to the
assignee hereof and hereby expressly incorporated by reference
herein.
FIELD OF THE DISCLOSURE
[0002] The present disclosure relates generally to Ultra-Low
Temperature and Ultra-High and Extreme-High Vacuum systems. In
particular, but not by way of limitation, the present disclosure
relates to systems, methods and apparatuses for a vacuum chamber
using one or more cryogenically cooled radiation shields to reduce
pressure within the chamber and/or to provide a cryogenic
environment for low temperature experiments.
FIGURES DESCRIPTION
[0003] FIG. 1 illustrates an ultra-high or extreme-high vacuum
system including a chamber, a radiation shield, a dedicated shield
cryostat, a cryostat with target, and optionally an experimental
tool.
[0004] FIG. 2 illustrates the same UHV or XHV vacuum system of FIG.
1 with the addition of a second dedicated cryostat and second
shield.
[0005] FIG. 3 illustrates the same UHV or XHV vacuum system of FIG.
2. with a two-stage dedicated shield cryostat.
[0006] FIG. 4 illustrates a UHV or XHV vacuum system with three
radiation shields, three shield cryostats, and a cryostat with
target.
[0007] FIG. 5 illustrates a prior art vacuum system with a
cryostat, vacuum pump, and target.
[0008] FIG. 6 illustrates another embodiment of a UHV chamber with
a single cooled radiation shield.
[0009] FIG. 7 illustrates a UHV or XHV vacuum system with two
radiation shields, a two-stage cryostat, an optional sorbent
material, a vacuum pump, and a generic apparatus.
[0010] FIG. 8 illustrates a UHV or XHV vacuum system with two
radiation shields, a two-stage cryostat, an optional sorbent
material, a vacuum pump, and a cryocooled target.
[0011] FIG. 9 shows a UHV or XHV vacuum system having two
cryocooled radiation shields via a cross sectional
illustration.
[0012] FIG. 10 shows another UHV or XHV vacuum system having two
cryocooled radiation shields via a cross sectional
illustration.
[0013] FIG. 11 illustrates a perspective view of a cross section of
a UHV or XHV vacuum system having two cryocooled radiation shields,
a cooled target, and a hemispherical ARPES analyzer.
[0014] FIG. 12 shows the prior art for a stand-alone hemispherical
analyzer and time-of-flight (TOF) analyzer.
[0015] FIG. 13 shows another prior art embodiment of a
hemispherical analyzer as it is typically connected to an UHV
chamber with a target.
[0016] FIG. 14 shows a new concept for a cryogenically cooled
hemispherical analyzer using a single-stage cryohead.
[0017] FIG. 15 shows a new concept for a cryogenically cooled TOF
analyzer, using a single-stage cryohead.
[0018] FIG. 16 shows another embodiment of a cryogenically cooled
hemispherical analyzer using a two-stage cryohead.
[0019] FIG. 17 shows another embodiment of a cryogenically cooled
TOF analyzer using a two-stage cryohead.
[0020] FIG. 18 shows a first embodiment of the 2-stage cooled
hemispherical analyzer of FIG. 16 attached to a cryogenically
cooled XHV vacuum chamber.
[0021] FIG. 19 shows a second embodiment of the 2-stage cooled
hemispherical analyzer of FIG. 16 attached to a cryogenically
cooled XHV vacuum chamber.
[0022] FIG. 20 shows an extension to the analyzer of FIG. 16 or 17
in which an extended detector is utilized
[0023] FIG. 21 is a perspective view of a cross section of the
analyzer in FIG. 16.
[0024] FIG. 22 is a perspective view of a cross section of the
analyzer in FIG. 17.
[0025] FIG. 23 is a perspective view of a cross section of the
analyzer and in FIG. 18.
[0026] FIG. 24 is a detailed view of the analyzer in FIG. 23
showing an embodiment of internal routing of the thermal busbar and
electrical breaks.
[0027] FIG. 25 shows various examples of analyzer and cryohead
configurations with both single and multiple cryoheads. From this
it can be seen that the disclosure intends to cover a wide variety
of experimental setups.
[0028] FIG. 26 is a 2-stage cooled hemispherical analyzer having a
first stage thermally coupled to a first set of one or more
electrodes and a second stage thermally coupled to a second set of
one or more electrodes.
[0029] FIG. 27 is a 2-stage cooled TOF analyzer having a first
stage thermally coupled to a first set of one or more electrodes
and a second stage thermally coupled to a second set of one or more
electrodes.
[0030] FIG. 28 is a schematic of the XHV chamber shown in FIGS. 2,
3, 7, and 8 showing exemplary relative sizes of the chamber to the
intermediate gaps. It also shows the vacuum levels and particle
species present in the intermediate gaps.
SUMMARY OF THE DISCLOSURE
[0031] The following presents a simplified summary relating to one
or more aspects and/or embodiments disclosed herein. As such, the
following summary should not be considered an extensive overview
relating to all contemplated aspects and/or embodiments, nor should
the following summary be regarded to identify key or critical
elements relating to all contemplated aspects and/or embodiments or
to delineate the scope associated with any particular aspect and/or
embodiment. Accordingly, the following summary has the sole purpose
to present certain concepts relating to one or more aspects and/or
embodiments relating to the mechanisms disclosed herein in a
simplified form to precede the detailed description presented
below.
[0032] Some embodiments of the disclosure may be characterized as
an ultra-high vacuum (UHV) or extreme-high vacuum (XHV) system
comprising a vacuum chamber, a target within the vacuum chamber,
two or more overlapping radiation shields arranged within an inner
vacuum space of a vacuum chamber, wherein the two or more
overlapping radiation shields surround at least a portion of the
target, a first cooling element unit thermally coupled to a first
radiation shield of the two or more overlapping radiation shields,
wherein the first cooling element unit is configured to reduce the
first radiation shield's temperature to at least <100K, a second
cooling element unit thermally coupled to a second radiation shield
of the two or more overlapping radiation shields, wherein the
second cooling element unit is configured to reduce the second
radiation shield's temperature to at least <25K, and a third
cooling element unit thermally coupled to the target, the third
cooling element unit thermally isolated from both the first
radiation shield and the second radiation shield, wherein the third
cooling element unit is configured to reduce the target's
temperature to at least <4K.
[0033] Other embodiments of the disclosure may be characterized as
a method for UHV or XHV comprising providing two or more
overlapping radiation shields within an inner vacuum space of a
vacuum chamber, the two or more overlapping radiation shields
covering at least 90% 4.pi. steradians around a target, thermally
coupling a first cooling element unit to a first of the two or more
overlapping radiation shields, thermally coupling a second cooling
element unit to a second of the two or more overlapping radiation
shields, thermally coupling a third cooling element unit to the
target, the third cooling element unit thermally isolated from the
first and second radiation shields, cooling the first radiation
shield to <100K, cooling the second radiation shield to <25K,
cooling the target to <4K, and interacting an elongated tool
with the target through one or more apertures in the first and
second radiation shields, while maintaining the at least 90% 4.pi.
steradians coverage around the target.
[0034] Some other embodiments of the disclosure may be
characterized as an apparatus for UHV or XHV comprising two or more
overlapping radiation shields within an inner vacuum space of a
vacuum chamber, wherein the two or more overlapping radiation
shields surround at least a portion of a target thereby blocking a
majority of blackbody radiation from reaching the target, means for
reducing a temperature of the first radiation shield to <100K,
means for reducing a temperature of the second radiation shield to
<25K, means for reducing a temperature of the target to <4K,
and wherein the means for reducing the temperature of the target is
thermally isolated from both of the means for reducing the
temperature of the first radiation shield and the means for
reducing the temperature of the second radiation shield, and means
for interacting with the target via one or more apertures in the
first and second radiation shields.
DESCRIPTION OF RELATED ART
[0035] Ultra-high vacuum (UHV) is the vacuum regime characterized
by pressures lower than about 10.sup.-7 pascal or 100 nanopascals
(10.sup.-9 mbar, .about.10.sup.-9 torr), while extreme-high vacuum
(XHV) is the regime characterized by pressures lower than about
10.sup.-10 pascal. UHV and XHV conditions are created by pumping
the gas out of a UHV/XHV chamber. At these low pressures the mean
free path of a gas molecule is greater than 40 km, so the gas is in
free molecular flow, and gas molecules may collide with the chamber
walls many times before colliding with each other. Thus, in some
aspects, almost all molecular interactions take place on various
surfaces in the chamber.
[0036] UHV/XHV conditions are integral to scientific research as
well as to modern technology. Surface science experiments often
require a chemically clean sample surface with the absence of any
unwanted adsorbates. Surface analysis tools such as X-ray
angle-resolved photoemission spectroscopy (ARPES), scanning
tunneling microscopy (STM), and low energy ion scattering require
UHV conditions for the transmission of electron or ion beams. For
the same reason, beam pipes in particle accelerators such as the
Large Hadron Collider are kept at UHV. In some cases, MBE growth
chambers require UHV conditions to remove contaminants that would
otherwise disrupt the pristine crystal during growth. In some
circumstances, ion traps for quantum information experiments may be
hindered by UHV levels not being low enough to prevent residual gas
particles from knocking ions out of the trap, thus shortening the
lifetime of the experiment.
[0037] Maintaining UHV/XHV conditions often involves the use of
specialized materials for the equipment that can withstand high
temperatures, as well as to maintain low out-gassing rates and
vapor pressures. In some cases, after venting the equipment to
atmosphere, the entire system may be heated above 100.degree. C. or
higher for many hours ("baking") to remove water and other trace
gases which adsorb on the surfaces of the chamber to prevent
materials from out-gassing particles into the vacuum during
operation. Thus, long-term operation (i.e., without baking) has yet
to be achieved.
[0038] In some cases, low pressures (or high vacuum) conditions are
often achieved by affixing one or more pumps to the chamber and
removing gas particles via pumping. Accordingly, the pressure or
vacuum is a function of the number and quality of the pumps over
the volume of the chamber, and a practical limit to the pressure
floor is the number and size of holes in the chamber that one can
make for addition of further pumps. In other words, for a given
chamber size, there is a limit to the pumping rate.
[0039] Low temperature conditions are often achieved via the use of
one or more radiation shields around the portion of the chamber
that is to reach the lowest temperatures (e.g., around a target,
such as a sample or an experimental tool). In some cases, these
radiation shields may be cooled (e.g., to 77K) to reduce their
thermal blackbody radiation and block radiation from outside the
shield (e.g., from the chamber walls, which are typically at around
300K). Accordingly, cooling efforts can be focused on removing
thermal energy from within the volume enclosed by the shield(s)
without having to fight thermal energy imparted from incident
thermal radiation. In some cases, the radiation shield(s) may be
cooled via direct contact with the same cryostat that is also used
to cool the target or experimental tool (e.g., see FIG. 5; see also
U.S. Pat. No. 5,339,650, US20100219832, U.S. Pat. No. 4,765,153).
For instance, the radiation shield may be thermally coupled to a
first stage of a two-stage cold head (also referred to as a
cryohead), where the second stage is thermally coupled to the
target and the second stage and target are enclosed by the
radiation shield. However, when one or more of these radiation
shields are thermally coupled to the cooling element, any changes
in temperature of the cooling element (e.g., intentional
temperature scans of the target) may be propagated into the
shield(s) leading to unwanted out-gassing of adsorbed particles,
instability of position via thermal expansion, or a
combination.
[0040] In some cases, such as in molecular beam epitaxy systems (or
MBEs), it may be desirable to remove residual gas particles from a
vacuum chamber during sample growth. To do this, MBEs may use one
or more panels cryogenically cooled (e.g., to 77K or lower), via
direct contact with liquid nitrogen, to "freeze out" residual gas
from the vacuum. In other words, when "warm" gas particles contact
the cryopanel(s), they condense and freeze on to the cryopanel(s),
thereby effectively removing the "warm" gas particles from the
chamber. In some cases, these cryopanels may need to be
periodically regenerated because they quickly become saturated with
particles under high gas loads and are no longer able to remove
particles from the vacuum. In some examples, regeneration involves
periodically bringing the cryopanel(s) to an elevated temperature
such that the frozen gases/particles can be purged from the
cryopanel and the system. This means that cryopanels can only
maintain desired vacuum pressures for limited periods of time.
[0041] In some other cases, angle-resolved photoemission
spectroscopy (ARPES) tools are often used to detect electron
emissions from cold targets held in a vacuum. While the targets are
often cooled, the ARPES tool is not, and thus acts as a source of
blackbody radiation and particle emission (e.g., due to inadequate
vacuum) to the target. If the target (or sample) is enclosed within
a cryogenically shielded vacuum chamber, then the `warm` surfaces
of the ARPES tool can act as (a) a heat source (via blackbody
radiation) to the cold target. In some cases, this blackbody
radiation may dominate the heat load on the target, impacting the
lowest target temperatures that can be obtained. (b) Additionally
or alternatively, since the vacuum levels within a warm analyzer
are not as good as they are inside the cryogenic vacuum chamber,
the ARPES tool may act as a gas contamination source aimed directly
at the cold target surface. In some aspects, this will decrease the
clean target lifetime (time period until a target has frozen out so
many particles from the vacuum that the target must be cleaned or
replaced) compared to the ideal situation. (c) In some other cases,
the ARPES tool may also act as a gas contamination source towards
certain gas-sensitive components in the system, for example in an
exchange-scattering spin detector.
DETAILED DESCRIPTION
[0042] The word "exemplary" is used herein to mean "serving as an
example, instance, or illustration." Any embodiment described
herein as "exemplary" is not necessarily to be construed as
preferred or advantageous over other embodiments.
[0043] In modern vacuum systems, the vacuum chamber and vacuum pump
are considered to be two separate devices that are attached to one
another. When the lowest possible vacuum is required, the vacuum
pump is chosen to be a type with the highest pumping speed and
lowest ultimate pressure available, such as a cryopump, where the
volume within the charcoal fins is surrounded by <15K in all
directions and the vacuum levels can attain XHV pressures. The
motivation behind this disclosure is to create a vacuum chamber
that generates the thermal and vacuum conditions that exist inside
of a cryopump by converting the chamber walls into cryopumps. This
is achieved by cryo-cooling radiation shields lining a substantial
portion, e.g., >90%, >95%, >99%, of the internal surface
area of the vacuum chamber. In the limit where the radiation
shields are nearly the same size as the chamber itself, almost all
of the internal surfaces become pumping surfaces, similar to inside
a cryopump. Furthermore, these radiation shields can be cooled by a
dedicated cryostat such that any internal experimental device or
process is completely decoupled from the shields and chamber. When
the chamber, radiation shields, and dedicated cryostat are viewed
as a single unit, the vacuum chamber and vacuum pump are no longer
separate devices because the chamber itself has become the
pump.
[0044] It is believed that radiation shields that are cooled by a
dedicated and isolated cryostat (e.g., a cryohead or coldhead) have
seldom or ever been cooled to less than 77K, have not utilized
closed-cycle refrigerators, and have not fully encased the target
space within a UHV/XHV chamber (i.e., where "fully encased" means
covering at least 90% 4.pi. steradians around the target).
[0045] To these ends, the present disclosure relates to systems,
methods and apparatuses for an UHV or extreme-high vacuum (XHV)
chamber using two or more overlapping cryogenically-cooled
radiation shields, thermally-isolated from the cooling element used
to cool the target or experimental tool. In other words, the herein
described systems, methods, and apparatuses include UHV or XHV
chambers using two or more radiation shields to increase the
pumping speed (via increased pumping surface area) of the chamber.
In one embodiment, two or more cryogenically-cooled radiation
shields can be arranged around the target or experimental tool,
where an outer radiation shield may be cooled to <77K, <70K,
<50K, or <35K and the inner radiation shield may be cooled to
<20K, <15K, <4.4 K, or <4K. In one aspect of the
disclosure, such a system can be applied to a particle detector
system (e.g., ARPES system) to help cool the detector surfaces,
provide improved black body radiation shielding, improved
electrical shielding, and/or decreased vacuum levels.
[0046] In some cases, the ability to cool radiation shields below
77K (e.g., 25K or 15K or 4K or 2.8K) may allow for effective
pumping of all gas species, including hydrogen, which is the
dominant gas preventing current UHV systems from attaining true XHV
pressures. The benefit of using closed-cycle refrigerators is
twofold: it allows for the use of sorbent materials on the colder
second-stage radiation shield, thus minimizing gas saturation and
reducing the frequency of regeneration (i.e., down-time seen with
cryopanels needed to regenerate the sorbent panels), and is more
convenient and long-term cost effective than the prior art because
it eliminates the need for expensive and wasteful liquid cryogens
such as liquid nitrogen (LN.sub.2) and liquid helium (LHe). In some
cases, fully enclosing the target space with a nearly perfect
vacuum pump (i.e., the radiation panel(s) act as pumps by freezing
out gases/particles) may allow the vacuum chamber to reach its
theoretical limit of maximum pumping speed, and therefore lowest
vacuum level. Furthermore, this full enclosure in radiation
shielding at low temperatures (e.g., sub-4K) also reduces the
thermal radiation load onto the target to virtually zero, allowing
the herein disclosed target cryostats to attain their minimum base
temperature while still maintaining access and manipulation of the
working environment for the experimental system via doors, shutters
and baffles that are actively cooled by the shields. As described
above, a "full enclosure" may be defined as the radiation shields
covering at least 90% 4.pi. steradians around the target.
[0047] FIG. 1 illustrates an ultra-high (UHV) or extreme-high
vacuum (XHV) system 100 including a chamber 102, a radiation shield
120, a dedicated shield cryostat 112, a second cryostat 110, a
sample (or target) 130, and optionally an experimental tool 140.
FIG. 2 illustrates the same UHV or XHV vacuum system with an
additional dedicated shield cryostat and additional radiation
shield. The sample 130 can be coupled to a "manipulator" of the
second cryostat 110 and thereby thermally coupled to the second
cryostat 110. In some cases, the sample can include a
superconducting circuit, or any other object to be operated at or
near absolute zero (i.e., .about.0K temperatures), and/or within
UHV or XHV.
[0048] As illustrated in FIG. 1, the second cryostat 110 may be
thermally isolated from the radiation shield 120, while the
dedicated shield cryostat 112 can be thermally coupled to the
radiation shield 120. In some circumstances, such an arrangement
allows the dedicated shield cryostat 112 to control a temperature
of the radiation shield 120 independently from changes in the
second cryostat 110. In the example shown, the sample 130 may be
completely isolated from atmosphere via the chamber 102. In some
cases, the chamber 102 may be composed of stainless steel, a high
permeability material such as mu metal, supermalloy, supermumetal,
or molybdenum permalloy. In some examples, the second cryostat 110
may cool the sample 130 to anywhere from room temperature to a
temperature of order 10 mK or below.
[0049] As illustrated, the radiation shield 120 may surround and
enclose a majority of the sample 130. Furthermore, although the
radiation shield 120 likely includes one or more openings or gaps
to allow for entry of the second cryostat 110, load locks, the
optional experimental tool 140, viewing windows, pump apertures,
etc., in an embodiment, the radiation shield 120 may cover greater
than 90%, or 95%, or 99%, or 99.5% 4.pi. steradians around the
sample 130.
[0050] In some cases, the dedicated shield cryostat 112 may be
thermally isolated from the second cryostat 110. Further, the
dedicated shield cryostat 112 can be coupled to the radiation
shield 120 so that the radiation shield is cooled to <77K,
<70K or <50K, or <45K. At these temperatures, one or more
gas species within the chamber 102 may freeze onto the radiation
shield 120. In such cases, the radiation shield 120 may act as a
distributed vacuum pump.
[0051] As illustrated, the radiation shield 120 may be of
comparable size to the chamber 102 such that they are separated by
a small gap (e.g., 0.5''-3''). The aspect ratio of this small gap
to the chamber size is preferably small enough (e.g., gap/chamber
<10%) such that gas molecules departing from the 300K chamber
wall are much more likely (e.g., >75%) to strike the shield and
freeze than they are to strike another 300K surface. Provided that
the shield is cold enough to pump certain gas species, this small
gap forces nearly all gas molecules of those species to be pumped
on their first or second departure rather than after hundreds or
even thousands of departures.
[0052] For the purposes of this disclosure, a radiation shield
(such as radiation shield 120) may be structurally different from
other types of vacuum shielding because its base material, physical
isolation, and surface finishes may be engineered to reduce thermal
radiation from the surrounding .about.300K chamber onto the sample,
or to greatly improve the effective pumping speed around the system
so as to enable much better vacuum conditions, or both. In some
examples, the base material can be selected to be an excellent
thermal conductor at temperatures below 300K (e.g., OFHC copper,
99.999% aluminum, etc.) so that incident heat can be removed
rapidly, allowing it to maintain a very low base temperature.
Additionally or alternatively, the shield can be physically
isolated from the warm chamber (e.g. via special mechanical
connections) to minimize heat leaks that might raise its base
temperature. In some other examples, the surface finishes can be
chosen to be highly reflective (or to have low emissivity) on the
outer surface (e.g., nickel or gold plating) to reflect away as
much 300K radiation as possible. Conversely, parts of the inner
surface (or any surface) can be chosen to be highly absorbent
(e.g., black finish) to prevent any radiation from reflecting
deeper into the system towards the target, while other parts can
also be made reflective to lower the emissivity toward the target.
It should be noted that a cryostat is just one example of a
"cooling element" that can be used to cool any of the herein
disclosed targets or the radiation shield(s). In an embodiment, one
or more of the cryostats mentioned in this disclosure can be
closed-cycle.
[0053] An associated benefit of this disclosure is that the cooled
radiation shield 120 that encases the workspace and sample 130
gives a drastic reduction in the thermal or "blackbody" radiation
that impinges the working space and sample. This follows from the
T.sup.4 scaling of blackbody radiation, so that a reduction of the
surrounding temperature from 300K (room temperature) to 10K is a
reduction of the thermal load by a factor of 30.sup.4 or 810,000.
This drastically reduced thermal load enables much more effective
and simpler designs for the separate second cryostat 110 (i.e., the
sample cryostat), for example allowing much colder attainable
temperatures, less liquid helium consumption for the cryogenic
manipulators, and/or the ability to design effective closed-cycle
manipulators of small size that significantly outperform cryogenic
manipulators in chambers without separately cooled radiation
shields. Applications for this include the development of XHV ARPES
(discussed later and see FIGS. 12-22) and STM chambers with
ultra-low temperature sample manipulators.
[0054] FIG. 6 illustrates another embodiment of a UHV chamber with
a single cooled radiation shield.
[0055] Although more than two radiation shields can be implemented,
and although the single-shield embodiments of FIGS. 1,6, 14, and 15
may not get cold enough to attain XHV vacuum levels, a
single-shield embodiment may have a cold enough shield to pump most
heavier gases, including water, which is one of the most
problematic to remove from vacuum systems. In some cases, a
single-shield embodiment using a closed-cycle cooling element
coupled to the radiation shield could be a potential replacement
for the liquid nitrogen-cooled shields in some current MBE
systems.
[0056] In other embodiments, two or more radiation shields can be
used, as seen in FIGS. 2, 3, 4, 7, 8, 9, 10, and 11. In such cases,
each radiation shield may be thermally coupled to its own cooling
element (e.g., a separate closed-cycle cryohead). For instance, in
FIG. 2, radiation shields 120, 122 are thermally coupled to
independent cooling elements of cryostats 112 and 114,
respectively. In other embodiments, a single two-stage cryostat
(e.g., cryostat 112 in FIG. 3 and the closed-cycle cryohead
(dual-stage) in FIGS. 7 and 8) can be used to cool each of two
radiation shields 120, 122, where the second stage (connecting to
the inner radiation shield 122) reaches temperatures of <25K, or
<15K, or <11K, or <4.4K, or <4K, or <3.5K, or
<2.8K. Typically temperatures below 15K can lead to XHV
operation.
[0057] It should be noted that for low-temperature applications and
XHV conditions it may be beneficial to cover all open ports, such
as those used for sample transfer, evaporation, viewing, etc., with
cooled shields (not illustrated), since even 1% of the 4.pi.
steradians that is at 300K and directed towards the cryogenic
sample may dominate over the other 99% that is at low temperatures
(e.g., 4K). In some aspects, these ports can be closed up or
partially closed up via two-piece "clamshell" designs (e.g., see
FIG. 9), where each of the pieces of the clamshell are cooled. In
some circumstances, the two-piece clamshell design may also include
separate cooled shutters for each port or for viewing, and a cooled
transparent window made of a thermally conductive material such as
sapphire.
[0058] In each of these embodiments, cooling overlapping radiation
shields and arranging them within the chamber and close to the
inside walls of the chamber may optimize the effective pumping
speed of the chamber, since vacuum pumps can be combined with the
effects of "freezing" or cryosorbing gases out of the vacuum onto
the radiation shields. In some cases, the radiation shields have a
finite surface area and will eventually saturate with sorbed gas,
such that any additional impingent gas on the shield will no longer
stick to the shield. In some examples, the total amount of sorbed
gas can be greatly increased by increasing the available cold
surface area, for example, by the addition of a sorbent material
that is thermally sunk (e.g., thermally coupled to, glued, affixed)
to one or more of the radiation shields (e.g., to the inner
radiation shield).
[0059] In one example, the sorbent material may be a micro-porous
material (e.g., activated coconut charcoal, molecular sieve,
anodized aluminum, etc.) that has a high effective surface area
(e.g., >1,000 m.sup.2 per gram), for instance, due to countless
microscopic cavities and interconnecting channels that penetrate
through the bulk of the material. By cooling the sorbent material
down to the temperature of the radiation shield, the total cold
pumping area of the shield can be increased (e.g., by 10,000 times
or more). For lighter gases that are still quite mobile on a
cryosorbed surface (e.g., H.sub.2), this also increases the amount
of time until the shield becomes saturated (e.g., by the same
10,000 times or more). For instance, the sorbed gas may migrate
along the surface of the radiation shield until it reaches the
sorbent material where it is captured.
[0060] It should be noted that cryopanels operating at higher
temperatures (e.g., 77K) are incompatible with the herein disclosed
sorbent materials since such "warm" cryopanels tend to pump
cryo-condensable gases (e.g., H.sub.2O, O.sub.2, CO.sub.2, etc.),
causing the surface of a sorbent to plug up before the inner pores
can be fully loaded. Thus, one of skill in the art would not
consider the use of a sorbent material on a cryopanel that was
cooled to roughly 77K. This challenge is overcome in this
disclosure via the use of two radiation shields, an outer shield
cooled to a warmer temperature (e.g., <100K) that freezes out
the cryo-condensable gases that would otherwise plug up a sorbent
material, and then an inner radiation shield cooled to a lower
temperature (e.g., <25K, or <15K) and thermally coupled to a
sorbent material that greatly increases the surface area of the
inner shield while not being susceptible to plugging. Examples can
be seen in FIGS. 7 and 8.
[0061] To further prevent the sorbent material from plugging as
noted above, the sorbent material can be enclosed within a separate
radiation shield (e.g., the outer radiation shield of FIGS. 7 and
8) so that all cryo-condensable gases will freeze onto the outer
radiation shield before they can migrate deeper toward the sorbent
on the inner shield (e.g., the inner radiation shield of FIGS. 7
and 8). The amount of coverage of the sorbent material on the
radiation shield can vary anywhere from small patches (e.g., as in
FIGS. 7 and 8) to full coverage of the entire shield, and the
sorbent material can be arranged on an inner surface of the shield
(or anywhere on the shield), an outer surface of the shield, or a
combination of both. Preferably, the arrangement of shields and
sorbent material enables most if not all cryo-condensable gases to
freeze onto an outermost radiation shield before they can reach the
inner radiation shield. In some examples, it may be beneficial to
thermally sink the sorbent to the coldest portion of the shield
(e.g., near the connection to the cryohead as in FIGS. 7 and 8), or
to the entire shield if all portions of the shield reach nearly the
same base temperature.
[0062] This arrangement of shields and sorbent material is shown as
a schematic in FIG. 28. The vacuum chamber 2809 encloses the outer
radiation shield 2806, which encloses the inner radiation shield
2802 with sorbent material 2812 lining the inside. The open circles
represent cryo-condensable gases 2803 (e.g., H.sub.2O, O.sub.2,
CO.sub.2, etc.) and the solid circles represent cryo-sorbable gases
2805 (e.g., H.sub.2, He, etc.). The sorbent material 2812 is
protected from cryo-condensable gases because they will encounter
and be captured by the outer shield before they can migrate deeper
through baffles, shutters, etc. (not pictured).
[0063] FIG. 28 also shows that the gaps between the shields are
chosen to be small enough (e.g., gap/chamber <10%) such that gas
molecules departing from any given surface are much more likely
(e.g., >75%) to strike a colder surface and freeze than they are
to strike the same surface from where they departed. This is true
for both the cryo-condensable gases 2803 and cryo-sorbable gases
2805 onto the outer and inner radiation shields 2806, 2802,
respectively. This ensures the maximum pumping speed for all gas
species.
[0064] As illustrated, the gaps between the shields 2806, 2802 also
define distinct vacuum volumes where different gas species and
vacuum levels are primarily present. The high vacuum (HV) volume is
typical of an un-baked chamber where all species are present,
except that the cryo-condensable gases 2803 are preferentially
covering the outer shield 2806. The ultra-high vacuum (UHV) volume
is typical of a baked chamber where the majority of the
cryo-condensable gases 2803 have been removed while the lighter
cryo-sorbable gases 2805 dominate. The inner most extreme-high
vacuum (XHV) volume offers orders-of-magnitude lower pressures than
a typical baked UHV chamber with the majority of all species
removed. This structure of cryo-shields and isolated gap volumes,
each with maximum possible pumping speed, allows for true UHV/XHV
levels to be achieved in an un-baked chamber so the requirements
for special high-temperature materials and lengthy baking periods
seen in the art are removed.
[0065] In some cases, the cold heads thermally coupled to the two
radiation shields may be turned on at different times. For
instance, a first cold head connected to the outer radiation shield
may be switched on, cooling the outer radiation shield to <100K.
In some cases, the outer radiation shield may pump out the
cryo-condensable gases from the vacuum chamber. Further, a second
cold head connected to the inner radiation shield affixed with the
sorbent may be switched on a particular duration of time after the
first cold head is switched on, or after the partial pressure of
one or more cryo-condensable gases in the vacuum chamber falls
below a threshold.
[0066] In some examples, the sorbent material can be supplied as
small solid chunks or pellets (e.g., less than 1 cm.sup.3). In such
cases, a tightly packed single layer of pieces may maximize the
cold pumping surface area while also ensuring that every piece gets
cold. The sorbent material can be electro-chemically applied to the
entire surface of the shield by means of anodization such that the
thermal contact between sorbent and shield is nearly perfect. In
some cases, electro-chemical application of the sorbent material
can be particularly effective, for instance, where the shield is
made from high conductivity aluminum. In such a case, the entire
surface can be anodized, creating a porous surface that will act as
the sorbent material, while maintaining essentially perfect thermal
contact between sorbent and shield. In some cases, XHV conditions
can be met when two radiation shields are used, one colder than the
other, where the colder inner shield includes a sorbent material,
and the sorbent material and inner shield are cooled to at least
15K.
[0067] A closed-cycle cold head is one example of the cooling
elements herein disclosed, and its major components can include an
expander, compressor, vacuum shroud, and radiation shield. In some
cases, a cold head may also be referred to as a cryohead, and the
two terms may be used interchangeably. The expander, commonly
referred to as the cold finger, is where the Gifford-McMahon,
pulse-tube, or any other style of cryogenic refrigeration cycle
takes place. In some cases, the expander may be connected to a
compressor by two gas lines and an electrical power cable. In some
examples, one of the gas lines may supply high pressure helium gas
to the expander, whereas the other gas line may return low pressure
helium gas from the expander. In such cases, the compressor may
provide the necessary helium gas flow rate at the high and low
pressure for the expander to convert into the desired refrigeration
capacity.
[0068] In some cases, the vacuum shroud can surround the cold end
of the expander in vacuum, limiting the heat load on the expander
caused by conduction and convection. In some cases, the radiation
shield can be actively cooled by the first stage of the expander
and can insulate the second stage from the room temperature
(.about.300K) thermal radiation being emitted from the vacuum
shroud. It should be noted that the radiation shield is not
required to be one continuous piece and may contain openings for
access to the inside; these openings preferably contain overlapping
components (e.g., baffles or shutters) so as to prevent direct
line-of-sight from the warm vacuum shroud to the colder second
stage.
[0069] In addition to these major components, the closed-cycle cold
head can be accompanied by several support systems. Typically,
laboratory systems will have an instrumentation skirt, which
provides a vacuum port and electrical feedthroughs, as well as a
temperature controller to measure and adjust the target
temperature. The system may also include electricity, cooling water
for the compressor, and one or more vacuum pumps for the target
space. As depicted in FIGS. 6, 7, and 8, the vacuum pump may be
connected to one end of the chamber, whereas the closed-cycle
cryohead may be connected to a different, second end of the
chamber.
[0070] Application to Charged Particle Analyzers
[0071] FIG. 12A shows a known Hemispherical ARPES Analyzer while
FIG. 12B shows a known Time-of-Flight (TOF) ARPES Analyzer. In both
cases the analyzer includes a detector at one end of a
vacuum-enclosed chamber, and electrodes running along a length of
the tool with an opening in the electrodes at the opposing end
where a target can be arranged. The electrodes can be biased to
control movement of electrons that leave the target and pass
through the electrode opening. A typical ARPES analyzer can include
an inner and outer Mu Metal shield and a stainless-steel (or some
other material) vacuum jacket. The analyzer electrodes can be
electrically isolated from each other.
[0072] FIG. 13 shows a known Hemispherical ARPES Analyzer coupled
to a UHV chamber. The UHV chamber can include a target and a
cryogenic target manipulator (i.e., to couple the target or sample
to a cold head) with a radiation shield. To reduce the unwanted
effects of the `warm` surfaces of the ARPES analyzer on the target,
the opening in the radiation shield adjacent the target is often
minimized.
[0073] Despite previous attempts to reduce black body radiation and
achieve low vacuums in ARPES vacuum chamber systems, application of
cooled radiation shields as discussed relative to FIGS. 1-11 can
further decrease the black body radiation and vacuum pressure of
known ARPES systems. In particular, a closed-cycle cryogenic cold
head(s) may be coupled to the analyzer electrodes and/or to one or
more radiation shields within the vacuum jacket to increase the
effective cold pumping surface area of any existing vacuum device.
Additionally, the cooled radiation shield(s) have the added
advantage of acting as a fully-enclosing electric "Faraday cage"
around the entire inner portion of the ARPES analyzer as well as
the target and target manipulator space, which reduces electronic
noise leaking into the analyzer from outside the vacuum chamber.
This also allows for a highly stable electronic reference point for
the internal portions of the experimental system. Although this
disclosure uses an ARPES analyzer as an exemplary charged particle
analyzer, this disclosure is equally applicable to any charged
particle analyzer system, an electrostatic analyzer, or any other
type of electron analyzer utilizing a vacuum.
[0074] FIG. 14 illustrates an embodiment of a cryogenically cooled
hemispherical ARPES analyzer 1400 and FIG. 15 illustrates an
embodiment of a cryogenically cooled TOF ARPES analyzer 1500. Both
embodiments can include a first closed-cycle single-stage cryohead
or cold head (e.g., cryohead 1401-a or cryohead 1501-a) coupled to
one or more analyzer electrodes (e.g., analyzer electrodes 1402 or
analyzer electrodes 1502). In some cases, this coupling can be made
through an electrical-isolation component (e.g., electrical break
1403 or electrical break 1503) which may be composed of a block of
sapphire or any other material that has a high thermal conductivity
but low electrical conductivity. In some cases, ARPES analyzer 1400
and TOF analyzer 1500 may include a detector 1405 or detector 1505
at one end of the analyzer (i.e., the end opposite the target). In
some cases, the ARPES analyzer or TOF analyzer may also include an
outer metal shield 1407 or 1507, an inner metal shield 1408 or
1508, and a vacuum jacket 1409 or 1509.
[0075] At the same time, since the analyzer electrodes are isolated
from each other, a thermally-conductive path (e.g., copper braid,
thermal rope, thermal strap, thermal busbar 1404 or 1504, or any
other rigid or flexible thermal path) can run between the cryohead
1401 or cryohead 1501 and the various electrodes 1402 or electrodes
1502 such that each electrode is cooled to the same temperature. In
some examples, the thermally-conductive path can be coupled to each
electrode through an electrical-isolation component (e.g., sapphire
electrical break), just as the cold head is. Other setups can also
be used to maintain thermal equilibrium between the various
electrodes, but electrical isolation therebetween.
[0076] In some cases, for instance, when a single cold head is
used, a thermally-conductive path may be needed between the two
electrodes of the hemispherical portion of the analyzer.
Optionally, in the hemispherical variation, a second cryohead
1401-b can be coupled to any or all of the electrodes in the
hemispherical portion of the analyzer. A thermal busbar 1404-b can
be used (as shown) to provide a thermal path between the second
cryohead 1401-b and all of the electrodes.
[0077] FIG. 16 illustrates an embodiment of a cryogenically cooled
hemispherical ARPES analyzer 1600 using a two-stage cryohead and a
cooled radiation shield. FIG. 17 illustrates an embodiment of a
cryogenically cooled TOF ARPES analyzer 1700 using a two-stage
cryohead and a cooled radiation shield. ARPES analyzer 1600 and TOF
ARPES analyzer 1700 may implement aspect(s) of ARPES analyzer 1400
and TOF ARPES analyzer 1500, as further described with reference to
FIGS. 14 and 15, respectively.
[0078] The hemispherical ARPES analyzer of FIG. 16 may include one
or more cryoheads 1601 (i.e., cryohead 1601-a, and cryohead
1601-b), one or more analyzer electrodes 1602, one or more
electrical breaks 1603, one or more thermal busbars 1604 (i.e.,
thermal busbar 1604-a, and thermal busbar 1604-b), a detector 1605
at one end of the ARPES analyzer, an outer radiation shield 1606,
an outer metal shield 1607, an inner metal shield 1608, and a
vacuum jacket 1609.
[0079] The TOF ARPES analyzer of FIG. 17 may include a dual-stage
cryohead 1701, one or more analyzer electrodes 1702, one or more
electrical breaks 1703, a thermal busbar 1704, a detector 1705 at
one end of the ARPES analyzer, an outer radiation shield 1706, an
outer metal shield 1707, an inner metal shield 1708, and a vacuum
jacket 1709.
[0080] In some cases, the inner and outer metal shields may be
examples of high magnetic permeability shields, and may be composed
of mu metal, supermalloy, supermumetal, molybdenum permalloy, or
any other material with a relative magnetic permeability above a
threshold (e.g., relative magnetic permeability >10,000). In
some cases, magnetic permeability may be related to the ability of
a material to support the formation of a magnetic field within
itself (i.e., the degree of magnetization that the material obtains
in response to an applied magnetic field). In some aspects,
materials with a high magnetic permeability can attract magnetic
fields and redirect magnetic energy through themselves, shielding
sensitive equipment or experimental setups. In some cases, the high
permeability shields deployed may allow for very low magnetic field
levels (e.g., <0.5 .mu.T, or <0.1 .mu.T) within the analyzer,
which is crucial for high resolution measurements of kinetic
energies of charged particles, such as electrons. In some cases,
electron emission from the sample (or target) may be facilitated
via ultraviolet (UV) or laser excitation.
[0081] In some cases, the cooled outer radiation shield 1606 or
outer radiation shield 1706 can be arranged inside both of the
outer metal shield 1607 or outer metal shield 1707 and inner metal
shield 1608 or inner metal shield 1708 and outside the electrodes
1602 or electrodes 1702. In these embodiments, the outer radiation
shield 1606 or 1706 can be cooled to a first temperature (e.g.,
<77K) while the electrodes 1602 or 1702 can be cooled to a
second temperature lower than the first temperature (e.g., <4K).
In this way, the electrodes 1602 or 1702 act as the cooled inner
shield seen for instance in FIGS. 3, 4, 7-11. In some cases, the
analyzer electrodes 1602 or 1702 may acts as cryosorbtion pumps,
since they are thermally coupled to the cold head. In some other
cases, a sorbent material may be affixed to the electrodes 1602 or
1702 to increase their cold pumping surface area, as further
described with reference to FIGS. 7 and 8.
[0082] In some cases, the two-stage cryohead 1601 or 1701 may allow
for colder temperatures and better overall thermal and vacuum
performance, as compared to a single stage cryohead 1401 or 1501,
at a cost of increased complexity.
[0083] FIG. 18 illustrates the embodiment of FIG. 16 coupled to a
cryogenically cooled extreme-high-vacuum (XHV) chamber 1800. A
hemispherical ARPES analyzer 1816 coupled to the XHV chamber 1800
may include one or more cryoheads 1801 (i.e., cryohead 1801-c, and
cryohead 1801-d), one or more analyzer electrodes 1802, one or more
electrical breaks 1803, one or more thermal busbars 1804 (i.e.,
thermal busbar 1804-a, and thermal busbar 1804-b), a detector 1805
at one end of the ARPES analyzer, an outer radiation shield 1806-b,
an outer metal shield 1807-b, an inner metal shield 1808-b, and a
vacuum jacket 1809. Furthermore, the XHV chamber 1800 may include
one or more cryoheads 1801 (i.e., cryohead 1801-a, and cryohead
1801-b), an outer metal shield 1807-a, an inner metal shield
1808-a, an outer radiation shield 1806-a, an inner radiation shield
1810, an optional radiation shield 1806-c surrounding the target
(or sample 1811), and an optional sorbent material 1812 affixed to
the inner radiation shield 1810. In some cases, the optional
sorbent material affixed to the inner radiation shield may serve to
optimize vacuum quality. In some cases, the inner and outer metal
shields may be examples of the high magnetic permeability shields,
as described with reference to FIG. 16. In some cases, the one or
more analyzer electrodes 1802 from the ARPES analyzer may extend
into the XHV chamber 1800.
[0084] In some cases, two-stage cryoheads such as cryohead 1801-c
and 1801-d can be used to cool the outer radiation shield 1806-b to
a first temperature and the electrodes 1802 to a second temperature
lower than the first temperature. In some cases, in addition to
guiding the charged particles towards the detector 1805, the
electrodes 1802 may also behave as the cooled inner shield seen for
instance in FIGS. 3, 4, 7-11.
[0085] Since the electrodes 1802 do not cover an entirety of the
XHV chamber 1800, the second radiation shield 1810 can be arranged
inside the outer radiation shield 1806-a of the XHV chamber 1800.
As illustrated, the XHV chamber 1800 may include two-stage cryohead
1801-a that cools both the inner and outer radiation shields of the
XHV chamber. Further, the XHV chamber may include a separate
two-stage cryohead 1801-b for the sample 1811, where the first
"warmer" stage of cryohead 1801-b is thermally coupled to the
radiation shield 1806-c surrounding the sample, and the second
"colder" stage is thermally coupled to the sample 1811.
[0086] In some cases, the outer radiation shield 1806-a of the XHV
chamber 1800 may be thermally coupled to or overlap with the outer
radiation shield 1806-b of the ARPES analyzer. For instance, three
different detailed views of such a connection or overlap are shown
in the inset of FIG. 18. These show an interleaving non-contacting
joint 1813, a tight-fitting overlapping joint 1814, and a flanged
joint 1815. Other joints and overlapping options are also possible
so long as they achieve a reduction in radiation leakage at this
joint.
[0087] FIG. 19 illustrates a variation on FIG. 18 where one of the
high permeability (e.g., Mu Metal) shields is removed and instead
of a vacuum jacket (e.g., stainless steel), a high permeability
vacuum jacket 1909 is used. In some cases, this may allow for a
more compact system with fewer layers of shielding within the
vacuum jacket 1909. In some cases, a hemispherical ARPES analyzer
1916 coupled to the XHV chamber 1900 may include: one or more
cryoheads 1901 (i.e., cryohead 1901-c, and cryohead 1901-d), one or
more analyzer electrodes 1902, one or more electrical breaks 1903,
one or more thermal busbars 1904 (i.e., thermal busbar 1904-a, and
thermal busbar 1904-b), a detector 1905 at one end of the ARPES
analyzer, an outer radiation shield 1906-b, an inner metal shield
1908-b, and the high permeability vacuum jacket 1909.
[0088] Furthermore, the XHV chamber 1900 may include one or more
cryoheads 1901 (i.e., cryohead 1901-a, and cryohead 1901-b), an
inner metal shield 1908-a, an outer radiation shield 1906-a, an
inner radiation shield 1910, an optional radiation shield 1906-c
surrounding the target (or sample 1911), and an optional sorbent
material 1912 affixed to the inner radiation shield 1910. In some
cases, the optional sorbent material may serve to optimize vacuum
quality in the XHV chamber 1900. In some cases, the inner metal
shield may be an example of the high permeability shield, as
described with reference to FIGS. 16 and 18. In some cases, the one
or more analyzer electrodes 1902 from the ARPES analyzer 1916 may
extended into the XHV chamber 1900.
[0089] In some case a high permeability coupler, such as a Mu Metal
coupler 1913, may be used to bridge the high permeability (e.g., Mu
Metal) gap between the ARPES analyzer 1916 and the XHV chamber
1900.
[0090] FIG. 20 illustrates another variation of FIG. 18 where an
optional extended detector 2005-b is utilized, such as a 3D
spin-resolved electron detector or 3D spin Very Low Energy Electron
Diffraction (VLEED). In some cases, the ARPES analyzer and XHV
chamber of FIG. 20 may implement aspects of FIGS. 7, 8, 14, 16,
and/or 18. FIG. 20 illustrates a hemispherical ARPES analyzer 2016
coupled to a XHV chamber 2000, and may include one or more
cryoheads 2001 (i.e., cryohead 2001-c, and cryohead 2001-d), one or
more analyzer electrodes 2002, one or more electrical breaks 2003,
one or more thermal busbars 2004 (i.e., thermal busbar 2004-a, and
thermal busbar 2004-b), a detector 2005-a at one end of the ARPES
analyzer 2016, the optional detector 2005-b, an outer radiation
shield 2006-b, an outer metal shield 2007-b, an inner metal shield
2008-b, and a vacuum jacket 2009.
[0091] Furthermore, the XHV chamber 2000 may include one or more
cryoheads 2001 (i.e., cryohead 2001-a, and cryohead 2001-b), an
outer metal shield 2007-a, an inner metal shield 2008-a, an outer
radiation shield 2006-a, an inner radiation shield 2010-a, an
optional radiation shield 2006-c surrounding the target (or sample
2011), and an optional sorbent material 2012 affixed to the inner
radiation shield 2010-a. In some cases, the optional sorbent
material affixed to the inner radiation shield may serve to
optimize vacuum quality. In some cases, the inner and outer metal
shields may be examples of the high permeability shields, as
described with reference to FIGS. 16 and 18. In some cases, the one
or more analyzer electrodes 2002 from the ARPES analyzer 2016 may
extended into the XHV chamber 2000.
[0092] In some cases, additional cooling may be added to this
system so as to have XHV conditions in the optional extended
detector 2005-b. In particular, the optional extended detector
2005-b may include an outer radiation shield 2006-d cooled by
either a cryohead 2001-e that is part of the extended detector, or
by the optional additional cryohead 2001-d coupled to the
hemispherical ARPES analyzer 2016. In some cases, the optional
extended detector 2005-b may also include an inner radiation shield
2010-b cooled by the extended detector's cryohead 2001-e.
[0093] It should be noted that throughout this disclosure, the
analyzer electrodes (e.g., analyzer electrodes 1402, 1502, 1602,
1702, 1802, 1902, or 2002) with slits are also thermally coupled to
a cryohead and may be cooled to <4K, or the same temperature as
the other electrodes, also making them effective radiation shields
for black-body radiation traveling within the electrodes towards
the target (or sample).
[0094] FIG. 21 is a perspective view of a cross section of the
analyzer in FIG. 14.
[0095] FIG. 22 is a perspective view of a cross section of the
analyzer in FIG. 15.
[0096] FIG. 23 is a perspective view of a cross section of the
ARPES system in FIG. 18, and FIG. 24 provides further details of
this embodiment. In particular, FIG. 24 shows an embodiment of
internal routing of the thermal busbar and electrical breaks. The
internal rigid and flexible busbars can be routed in many different
ways to accommodate a variety of cold head configurations and
numbers.
[0097] In some cases, the ARPES system in FIG. 24 may include a
dual stage closed cycle cryohead 2401-a, one or more analyzer
electrodes 2402, a thermal busbar 2404-a, one or more electrical
isolators 2403 (e.g., electrical isolator 2403-a) between the
thermal busbar 2404-a and analyzer electrodes 2402, a detector
2405, an outer radiation shield 2406, an inner radiation shield
2410, an outer metal shield 2407 (e.g., Mu Metal), an inner metal
shield 2408 (e.g., Mu Metal), one or more flexible thermal busbars
2409 (e.g., copper braid, or rope), and a slit carousel 2411.
Furthermore, the linear section of the ARPES system, i.e., closer
to the target or sample, may comprise: a second closed-cycle
cryohead 2401-b, a thermal busbar 2404-b, and one or more
electrical isolators 2403 (e.g., electrical isolators 2403-b). In
some cases, the electrical isolators 2403 may be composed of a
thermally conductive, and electrically isolating material, such as
Sapphire.
[0098] FIG. 25 shows various examples of hemispherical ARPES
analyzer and cryohead configurations with both single and multiple
cryoheads. From this it can be seen that the disclosure intends to
cover a wide variety of experimental setups.
[0099] FIG. 26 illustrates a 2-stage cooled hemispherical analyzer
2616, while FIG. 27 illustrates a 2-stage cooled TOF analyzer 2716.
A primary consideration in such analyzers is to minimize radiation
and heat at the sample. This variation recognizes that the entire
analyzer does not need to be maintained at minimum temperatures in
order to achieve nearly the same result as some of the embodiments
shown earlier. Accordingly, this variation seeks to reduce the
number of cryoheads as well as radiation shields while still
maintaining XHV conditions at the sample. To do this, a single
two-stage cryohead 2601 or cryohead 2701 is used where the cooler
second stage (e.g., 3K) is thermally coupled to one or more
analyzer electrodes 2602-a or electrodes 2702-a in the linear
(straight) portion of the analyzer, as well as an electrode with an
intermediate slit 2610-a or 2710 within this linear section. A
second stage thermal busbar 2604-a or other thermally conductive
path can thermally couple the second stage of the cryohead 2601 or
2701 to electrodes 2602-a or 2702-a in the linear section.
[0100] Toward the detector 2605 or detector 2705, the first stage
(e.g., 45K) of the two-stage cryohead can thermally couple to one
or more electrodes 2602-b or 2702-b that are on the detector-side
of the electrodes with a slit 2610-a or 2710. This connection may
be made via a thermal busbar 2604-b or 2704-b. In other words, the
second colder stage of the cryohead can cool a first set of one or
more analyzer electrodes (i.e., analyzer electrodes 2602-a or
2702-a) closer to the sample to a first temperature, and the first
stage of the cryohead can cool a second set of one or more analyzer
electrodes (i.e., analyzer electrodes 2602-b or 2702-b) closer to
the detector 2605 or 2705 to a second temperature higher than the
first temperature. It should be noted that the first stage of the
cryohead is thermally coupled to both the second set of one or more
electrodes 2602-b or 2702-b and also an outer radiation shield 2606
or outer radiation shield 2706.
[0101] This variation does not need to cool the electrodes 2602-c
in the hemispherical portion of the analyzer since an electrode (or
radiation shield) with slit 2610-b blocks most of the 300K
radiation from the hemispherical portion as well as pumps most
particles attempting to enter the linear portion from the
hemispherical portion.
[0102] Said another way, as seen from the sample, the second colder
stage of the cryohead can cool as far as the electrode with slit
2610-a or 2710 in order for nearly all black-body solid angle to be
blocked at the sample. The first stage (i.e., warmer stage) of the
cryohead can again cool the outer radiation shield 2606 or 2706 as
in previous embodiments, but here also cool electrodes 2602-b or
2702-b between the electrode with slit 2610-a or 2710 and either:
the electrode (or radiation shield) with slit 2610-b (hemispherical
version); or the detector (time-of-flight version). Where the
second radiation shield with slit 2610-b is implemented, the first
stage can also thermally couple to this shield. This configuration
is almost as effective as cooling every element because the only
300K radiation and gas load that hits the sample must pass through
two narrow slits spaced far apart so the solid angle is extremely
low (in the hemispherical variation) and must pass through one
narrow slit spaced far from the sample in the TOF variation, so the
solid angle is also very low.
[0103] It should be noted that the location of the thermal path
between the first stage of the cryohead and the second set of one
or more electrodes 2602-b or 2702-b closer to the detector, or
between the second radiation shield with slit 2610-b and the second
set of one or more electrodes 2602-b or 2702-b can vary and does
not have to comport with the illustrated locations in FIG. 26 or
27.
[0104] As used herein, the recitation of "at least one of A, B and
C" is intended to mean "either A, B, C or any combination of A, B
and C." The previous description of the disclosed embodiments is
provided to enable any person skilled in the art to make or use the
present disclosure. Various modifications to these embodiments will
be readily apparent to those skilled in the art, and the generic
principles defined herein may be applied to other embodiments
without departing from the spirit or scope of the disclosure. Thus,
the present disclosure is not intended to be limited to the
embodiments shown herein but is to be accorded the widest scope
consistent with the principles and novel features disclosed
herein.
* * * * *